Financial Instruments (Foreign Currency Risk Management) (Details)	Mar. 31, 2012 USD ($)	Mar. 31, 2012 Eurobond 20% [Member] USD ($)	Mar. 31, 2012 Eurobond 20% [Member] EUR (€)	Sep. 30, 2011 Eurobond 20% [Member] USD ($)	Sep. 30, 2011 Eurobond 20% [Member] EUR (€)	Mar. 31, 2012 Eurobond 80% [Member] USD ($)	Mar. 31, 2012 Eurobond 80% [Member] EUR (€)	Sep. 30, 2011 Eurobond 80% [Member] USD ($)	Sep. 30, 2011 Eurobond 80% [Member] EUR (€)	Mar. 31, 2012 Forward Foreign Currency Contracts [Member] USD ($)		Sep. 30, 2011 Forward Foreign Currency Contracts [Member] USD ($)
Debt Instrument [Line Items]
Long-term debt	$ 175,000,000	$ 175,000,000				$ 175,000,000
Percentage of debt hedged by cross currency swaps		20.00%	20.00%			80.00%	80.00%
Notional amount of foreign currency derivative instruments not designated as hedging instruments		35,000,000	31,000,000	35,000,000	31,000,000	140,000,000	124,000,000	140,000,000	124,000,000	67,000,000	[1]	54,000,000	[1]
Notional Amount of Foreign Currency Cash Flow Hedge Derivatives										$ 7,000,000	[2]	$ 12,000,000	[2]

[1] Cabot's forward foreign exchange contracts are denominated primarily in the Australian dollar, British pound sterling, Canadian dollar, Euro, Japanese yen, and Malaysian ringgit.

[2] Cabot's forward foreign exchange contracts designated as cash flow hedges are denominated in Japanese yen and are presented in their USD equivalent in the table above.